1. Field of the Invention
The present invention relates to a photomask unit, an exposing method and a method for manufacturing a semiconductor device.
2. Description of the Related Art
Photomask unit normally has a mask substrate (photomask main body) having formed thereon a mask pattern, and a pellicle provided to the mask substrate. The pellicle can protect a pattern forming surface of the mask substrate.
A film thickness, a refractive index and an extinction coefficient of the pellicle are adjusted so that transmittance of exposure light becomes as high as possible (for example, see Jpn. Pat. Appln. KOKAI Publication No. 7-199451). The refractive index and the extinction coefficient are called optical constants as long as there is no need to specify either of them. Normally, the film thickness and the refractive index are adjusted so that the transmittance of light incident on the pellicle vertically becomes as high as possible.
According to the miniaturization of mask patterns, however, an exposing method using off-axis illumination is required. In the off-axis illumination, exposure light is incident on the pellicle off-axially. For this reason, a conventional pellicle where only vertical incident light is taken into consideration cannot provide sufficient transmittance.
In such a manner, according to the miniaturization of the mask patterns, a decrease in the transmittance of the pellicles becomes a serious problem. For this reason, photomask units having a pellicle optimized for the off-axis illumination are desired.